"Proof marks" (or something similar) added at the end. Since the exterior shell isn't metal, maybe it's done with heat? I thought the camo wrap guns had a "window" for serial numbers and the like. Maybe they don't do that anymore, or at least on the SN, or not for that marking.
I suppose you could put a thin coat of clear silicone like a gasket between the two, that would help with any scuffing. But I don't think there is any really. It's not moving around to cause wear.
I have an as new mount with the rail that needs a new home FWIW.